1. Field of the Invention
The present invention relates to a pallet exchange apparatus for exchanging pallets on which workpieces are mounted, between a workpiece loading-and-unloading station and a workpiece machining station of a machine tool.
2. Prior Art of the Invention
Referring to FIG. 1, in a conventional pallet exchange apparatus, an arm 51 is supported by a bed 50 through a drive shaft to be horizontally rotated by a rack-and-pinion mechanism 52. The rack-and-pinion mechanism 52 is arranged at the lower end of the drive shaft of the arm 51 and operated by a hydraulic cylinder. The arm 51 is arranged between a workpiece loading-and-unloading station A and a workpiece machining station B at which pallets 53 are located. Workpieces are fixed to the pallets 53 by the use of jigs 57. Upon the completion of machining of a workpiece, the pallets 53 are raised by cylinder devices 55 to be engaged with both ends of the arm 51 through hook portions 56 of the pallets 56. The arm 51 is rotated by 180 degrees and the pallets 53 are also rotated therewith along arcuate guide rails, which are not shown. In such a way, the pallets 53 are exchanged between the workpiece loading-and-unloading station A and the workpiece machining station B. After the exchanging operation, the pallets 53 are lowered by the cylinder device 55 and disengaged from the arm 51. Then, the pallet 53 at the workpiece machining station B is rotated on a turntable 54 mounted on the bed 1 by a predetermined angle.
Another pallet exchange apparatus is illustrated in FIG. 2. After the machining operation, the pallet 53 at the workpiece machining station B is lowered by the cylinder device 55 to be engaged with the arm 51 through first knock pins 58 formed on the arm 51. The arm 51 is then rotated by 180 degrees so as to exchange the pallet 53 with another pallet 53. After that, the pallet 53 at the workpiece machining station B is raised by the cylinder devices 55 so that the first knock pins 58 are pulled out from the pallet 53. At the same time, the pallet 53 is engaged with the turntable 54 through second knock pins 59 which are attached to the turntable 54.
In both pallet exchange apparatuses, the jigs 57 are disposed on the pallets 53 for clamping the workpieces. Since the pallets 53 are not securely held by the arm 51, when the arm 51 is rotated at a high speed, the jigs 57 become unstable due to the centrifugal force and inertia force generated during the acceleration and deceleration of the arm 51. In case where the hooks 56 and the first knock pins 58 in the apparatuses shown in FIGS. 1 and 2, respectively, are extended in the vertical direction to solve the problem, ascend-and-descend stroke of the pallets 53 becomes large and the time required for exchanging pallets 53 also becomes long.
In the pallet exchange apparatus shown in FIG. 1, there is another problem that the pallets 53 make a noise when the pallets 53 move along the arcuate guide rails. Consequently, it is difficult to move the pallets 53 at a high speed, whereby there is a limit to shorten the machining cycle time.